1. Field of the Invention
The invention relates to method and system for borehole sonic measurement, more particularly, the invention relates to method and system for analyzing the formation rock in-situ stress.
2. Background Art
Extracting quantitative information of the formation rock stresses from borehole log measurements is fundamental to the analysis and prediction of geo-mechanical problems encountered in the petroleum industry. Today there is no direct measurement to fully characterize the formation rock geo-stresses tensor (three principal stresses and three angles to describe the directions). For most of cases it can be reasonably assumed that the vertical stress is one principal stress, so, there are four parameters to describe the geo-stresses: vertical stress, minimum horizontal, maximum horizontal stresses and the azimuth of minimum horizontal stress. Thus, the in-situ stresses of a formation can be represented by the vertical stress, maximum horizontal stress, minimum horizontal stress, azimuth of minimum horizontal stress and the pore pressure. The vertical stress may be estimated from an integral of the density log, while the minimum horizontal stress can be estimated using fracturing or leak-off test data, and its direction from borehole caliper or images analysis. However, the maximum horizontal stress is more difficult to estimate, the conventional approach is to use some correlations such as the pore-elastic strain correlation, or the approximations such as equating the maximum horizontal stress to some multiple of the minimum horizontal stress.
However, these kinds of correlations for estimating maximum horizontal stress are always associated with big uncertainty. In addition, there is no correlation or model available to interpret directly from the vertical and minimum horizontal stresses logging to that of the maximum horizontal stress. Recent developments in sonic logging involve measuring the formation rock anisotropic wave velocities induced by in-situ stress anisotropy, which have been discussed in the following patents and publications. (1) U.S. Pat. No. 5,838,633 issued to Sinha et al., discloses a method for formation stress magnitude and formation non-linear parameters by using a high frequency sonic signal and a low frequency sonic signal; (2) U.S. patent application Ser. No. 12/413,178, Method to estimate subsurface principal stress directions and ellipsoid shape factor R from borehole sonic log anisotropy directions and image log failure directions, by Romain Prioul, et al.; (3) Colin M. Sayers, Sensitivity of elastic-wave velocities to stress changes in sandstones, The Leading Edge, December 2005, 1262-1266, discloses the relation between elastic-wave velocity change and the in-situ stress change; (4) U.S. Pat. No. 6,904,365 issued to Bratton, T. R. et al, discloses a method for determining formation stress parameter by using radial stress profile derived from the logging data and formation models; (5) Sarkar, D., Bakulin, A., and Kranz, R., 2003, Anisotropic inversion of seismic data for stressed media: Theory and a physical modeling study on Berea Sandstone: Geophysics, 68, 690-704, discusses modeling the relationship between the magnitude of the principal stresses and anisotropic parameters; (6) Prioul, R., A. Bakulin, V. Bakulin (2004), Non-linear rock physics model for estimation of 3-D subsurface stress in anisotropic formations: Theory and laboratory verification, Geophysics, Vol. 69, pp. 415-425 and (7) U.S. Pat. No. 6,714,873, System and method for estimating subsurface principal stresses from seismic reflection data, issued to Bakulin, et al., discusses determining formation stress characteristics by using the relationship between the measured seismic data and the known rock properties and elastic stiffness and/or sonic velocity.
Accurate estimation of geological formation stresses is desirable in the hydrocarbon production business, because formation stress determination is considered critical for hydrocarbon production planning, as well as providing prediction of sanding and borehole stability. As a result, there is a growing demand in the art for accurate estimation or determination of formation stresses.